Extraction of Bond Coat Mechanical Properties using High- Temperature Digital Image Correlation

Abstract

Gas turbine engines used in aircraft or power generation widely use nickel-based superalloys in their hot sections such as combustion chambers, turbine blades, etc. These components in the hot section use thermal barrier coating (TBC) that reduces the temperature experienced by the substrate. A typical TBC is made of two layers: an oxidation-resistant bond coat such as diffusion aluminide or overlay MCrAlY coating, and a ceramic top coat, typically yittria stabilized zirconia (7YSZ). While oxidation and corrosion protection are the key functions of bond coats, their mechanical behaviour plays a key role in the spallation life of the TBC, particularly in thermal cycling scenarios like those experienced in aircraft engines. The cyclic thermal loading leads to undulation on the bond coat surface known as rumpling, which is driven by the thermo-mechanical properties of the bond coat. However, limited information on these properties is available in the literature. Attempts to measure these properties have been inhibited by the limited thickness of these coatings. Studies to measure their properties using ingots cast with the same chemical composition are undermined by the fact that the microstructure of the bond coat is substantially different than that of the casting. In the present work, a small-scale testing method based on digital image correlation (DIC) has been developed to perform this study. This testing method is first validated by obtaining the properties of commercially pure copper that showed excellent correlation. It was then used to characterize the thermo-mechanical properties of NiCoCrAlY (overlay type) of bond coats. Properties like CTE, Young’s Modulus, Poisson’s ratio, and creep parameters were obtained. Young’s modulus and CTE of similar coatings available in the literature show a very good correlation, however, the creep parameters show significant differences. The possible reasons for these differences have been discussed. The deposition process used for coating can also play a role in the mechanical properties. A comparative study of the mechanical behaviour of bond coats obtained from the same feedstock but two different processes of deposition: 1) High-Velocity Oxy-Fuel (HVOF), and 2) Vacuum Plasma Spray (VPS) process was performed. The study shows a significant difference in the elastic-plastic behaviour of the bond coats obtained from the two processes indicating that the deposition process plays an important role in the mechanical behaviour of bond coats. Finally, to understand the difference in the behaviour of the two types of coating, microstructural studies were performed. These studies indicate a significantly larger presence of oxides in coatings deposited using the HVOF process as compared to the VPS process. These oxide particles increase the brittleness of HVOF coatings which leads to lower plastic deformation in them as compared to the VPS coatings.